2
$\begingroup$

I'm doing the GRE Biology practice test, and question #1 asks:

enter image description here

The answer is E. I understand why II and IV are polar, but why is $\ce{R-S-H}$ polar? R is a side chain to the sulfur, sulfur's valence shell is full, and sulfur is highly electronegative. But without knowing what $\ce{R}$ is, how do we know the molecule's total polarity?

Improve this question
$\endgroup$
4
  • 6
    $\begingroup$ The question didn't ask about a molecule's "total polarity," it asked which would form Hydrogen bonds. The hydrogen on a thiol group (R-S-H) can form hydrogen bonds with other groups like alcohols and amines. Thiols are sparingly water soluble so III is a bit strange but not impossible. Thiols only form weak hydrogen bonds between themselves. $\endgroup$
    – MaxW
    Commented Jul 13, 2018 at 18:09
  • 1
    $\begingroup$ @MaxW That answers it, thanks! If you'd like an accepted answer, please convert your comment to an answer and I'll accept. Either way, thank you. $\endgroup$
    – user61822
    Commented Jul 13, 2018 at 21:04
  • $\begingroup$ @freeradical Could you please update the question title - in your own words - to reflect the actual question being asked? $\endgroup$
    – Gaurang Tandon
    Commented Jul 14, 2018 at 3:13
  • $\begingroup$ @GaurangTandon Sorry, just saw this. I've updated the title, thanks. $\endgroup$
    – user61822
    Commented Aug 6, 2018 at 0:58

1 Answer 1

Reset to default
3
$\begingroup$

Actually it's all four. See here for I. The article describes a hydrogen bonding interaction between methane and water. It's weaker than the "usual" hydrogen bonds but still contributes to forming methane-water clathrates.

The larger lesson here is that hydrogen bonding is not just the electrostatic interaction often described in textbooks. It has a molecular orbital component that works even with primary bonds of low polarity.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ Fascinating, thanks! I'll report this to the GRE folks. I also recently read that the electronegativities of Hydrogen and Carbon are not equal (2.2 and 2.55 respectively), so it makes sense that even Methane is slightly polar and thus can make hydrogen bonds. $\endgroup$
    – user61822
    Commented Aug 8, 2018 at 3:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.